A common device used to define the radial spatial extent of an optical beam, or other form of electromagnetic wave, is to shear a portion of the beam with a circular aperture. The beam emerging from the aperture is defined spatially, but, as the distance from the aperture increases, large intensity variations occur across the width of the beam. The diffraction effect produced on the beam as it passes through the circular aperture causes these intensity variations. A particular example of this effect occurs with a uniformly illuminated circular aperture. After the beam passes through the circular aperture, a peak intensity arises at the center of the beam, and the intensity surrounding the center decreases. This intensity peak may be as much as a factor of four larger than the average intensity of the remainder of the beam.
In many commercial and scientific applications, one desires to obtain an optical beam such as a laser beam, in which spatial variations such as this intensity peak are minimized or absent. Such a requirement is particularly pertinent in high power solid state laser systems wherein self-focusing can destroy the lasing device.